Human eye is essentially made up of three basic layers of tissue divided into three chambers as shown in FIG. 1. The sclera (12) surrounds the lens (3) except at the cornea (1), which is the transparent tissue and the exterior surface of the eye through which light first enters the eye. The iris (2) is a coloured, contractible membrane that controls the amount of light entering the eye by changing the size of the circular aperture at its center (the pupil). The ocular or crystalline lens (3) (herein after referred to as lens) is located just posterior to the iris (2). The lens comprises transparent gel-like proteins in a transparent capsule bag (not shown). Generally the lens changes shape through the action of the ciliary muscle (8) to allow for focusing of a visual image. A neural feedback mechanism from the brain allows the ciliary muscle (8), acting through the attachment of the zonules (11), to change the shape of the ocular lens. Generally, sight occurs when light enters the eye through the cornea (1) and pupil, then proceeds past the ocular lens (3) through the vitreous (10) along the visual axis (4) (herein after referred to as central axis of the lens), strikes the retina (5) at the back of the eye, forming an image at the macula (6) that is transferred by the optic nerve (7) to the brain. The space between the cornea and the retina is filled with a liquid called the aqueous in the anterior chamber (9) and the vitreous (10), a gel-like, clear substance posterior to the lens.
Usually, the eye converges light using two primary elements, the cornea and lens, onto the retina for detailed vision. 75% of the total 60 diopter focusing power of the eye is provided by the first element i.e. the convex outer surface of the cornea. The remaining 25% is provided by the crystalline lens, the second element.
The effective focal length of the human eye must be adjusted to keep the image of the object focused as sharply as possible on the retina/macula. This change in effective focal length is known as accommodation and is accomplished in the eye by varying the shape of the crystalline lens. This is necessary for the human eye to have clear vision of objects at different distances. Generally speaking, in the unaccommodated normal vision, the curvature of the lens is such that distant objects are sharply imaged on the retina/macula. In the unaccommodated eye, close objects are not sharply focused on the retina/macula and their images lie behind the retinal surface. In order to visualize a near object clearly, the curvature of the crystalline lens is increased, thereby increasing its refractive power and causing the image of the near object to fall on the retina/macula.
The lens is a bi-convex structure suspended by ligamentous zonules attached to an annular ciliary muscle. While the lens contributes only 25% of the focusing power, the primary purpose is to allow proper and perfect focusing of divergent light reflected from near objects as well as from far objects. This ability to focus on far way objects as well as to focus on near objects by change in dimensions of the lens is called accommodation.
Lens fibers grow through out the life by the elongation and differentiation of epithelial cells circumferentially at the equator of lens, which results in internalization of previously, formed protein fibers. The net effect is that the older protein fibers are always found towards the nucleus and the younger protein fibers towards the cortex.
Presbyopia is attributed to continued proliferation of lamellar lens cells. As the lens ages, it becomes less elastic due to change in the lens's curvature from continual growth and therefore its ability to change its curvature in response to the contraction and relaxation of ciliary muscles is reduced/inhibited and results in vision defects such as but not limited to presbyopia with age. Presbyopia also causes circumlental crowding in the sub-cialiary region of the eye including crowding of the zonular fibers as show in FIG. 2 along resulting in loss in power of the cialary muscles.
Therefore, restoration of accommodation ability of lens has always been a topic of research. Bifocal and other multifocal spectacles and contact lens more commonly use to solve the focal problem of presbyopia. A common disadvantage of these devices is that the poorly focused portions of the image reduce the contrast of the focused part of the image.
Other method included a phaco-ersatz procedure to remove a lens from the lens capsule of an eye and a liquid is then introduced into the lens capsule, followed by inserting a supplemental endo-capsular lens into the lens capsule.
As noted from the prior art, U.S. Pat. No. 6,322,556 provides a method for selective removal of ocular lens tissue of human eye for the correction of vision defects including myopia, hyperopia or presbyopia by means of laser ablation of such a selected region. As per the preferred embodiment of the said U S patent the more centrally located older cortical and/or nuclear fibers be ablated since the width of the nucleus remains relatively constant with age, whereas that of cortex increases.
So also the said patent emphasize that the reduction in thickness of lens is responsible to restore accommodation. Further, there is possibility of exposure of the retina near the macula or macula to the laser beam due to leaking of laser beam through the lens during the ablation process, which may harm the retina near macula.
However, in respect of photo ablation of the lens, it should be appreciated that the reprofiling of lens by reducing the thickness of lens is not the determinative factor for restoration of accommodation. Therefore, to enhance the ability of the ciliary muscle to constrict and relax via zonular ligaments is the topic of research for the scholars.
Therefore there is a need to provide a method and system for photoablation of tissues/fibers within the lens for correcting vision problems, particularly, associated with presbyopia by substantially reducing damage to the lens and other parts of eye.